Set default for RPI command to return help info

RaspberryPi/apple2driver/handlers/exec.go

@@ -32,6 +32,9 @@ func ExecCommand() {
 		linuxCommand = strings.ToLower(linuxCommand)
 	}
 	linuxCommand = strings.Trim(linuxCommand, " ")
+	if linuxCommand == "" {
+		linuxCommand = "a2help"
+	}
 	fmt.Printf("Command to run: %s\n", linuxCommand)
 	if strings.HasPrefix(linuxCommand, "cd ") {
 		workingDirectory = strings.Replace(linuxCommand, "cd ", "", 1)